Quantifying sources, transport, deposition, and radiative forcing of black carbon in the Northern Hemisphere mid-latitudes

Abstract:

We use the Community Atmosphere Model version 5 (CAM5) with a novel explicit source-tagging technique to characterize the transport of BC originating from various geographical regions and sectors to the Himalayas and Tibetan Plateau (HTP) and the Western North America (WNA). The results show that BC source attribution depends on season and location in the HTP. The largest contribution to annual mean BC burden and surface deposition in the entire HTP region is from biofuel and biomass (BB) emissions in South Asia, followed by fossil fuel (FF) emissions from South Asia, then FF from East Asia. The same roles hold for all the seasonal means except for the summer when East Asia FF becomes more important. The annual mean radiative forcing (0.42 W m^-2) due to BC in snow outweighs the BC dimming effect (-0.3 W m^-2) at the surface over the HTP. We further evaluate the CAM5 results against the Positive Matrix Factorization (PMF) analysis of BC in snow over the WNA where an epic survey of BC in snow was conducted during the winter of 2013. CAM5 has a significant low bias in predicting BC in snow but only a small low bias in atmospheric BC concentrations. Local sources contribute more to near-surface atmospheric BC and to deposition than distant sources, while the latter are more important in the middle and upper troposphere. FF is the dominant local source type for BC burden and deposition, while for all distant source regions BB contribution is larger than FF. While CAM5 is qualitatively consistent with the PMF analysis with respect to partitioning of BC originating from BB and FF emissions, it significantly underestimates the relative contribution of BB. In addition to a possible low bias in BB emissions used in the simulation, CAM5 is likely missing a significant source of snow darkening from local soil found in the observations.